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Nobel Lecture

Nobel Lecture, May 6, 1966

Development of Quantum Electrodynamics

Personal recollections

(1) In 1932, when I started my
research career as an assistant to Nishina, Dirac published a paper
in the Proceedings of the Royal Society, London1. In this paper, he discussed the
formulation of relativistic quantum mechanics, especially that of
electrons interacting with the electromagnetic field. At that
time a comprehensive theory of this interaction had been formally
completed by Heisenberg and Pauli2, but Dirac was not satisfied with this
theory and tried to construct a new theory from a different point
of view. Heisenberg and Pauli regarded the (electromagnetic)
field itself as a dynamical system amenable to the Hamiltonian
treatment; its interaction with particles could be described by
an interaction energy, so that the usual method of Hamiltonian
quantum mechanics could be applied. On the other hand, Dirac
thought that the field and the particles should play essentially
different roles. That is to say, according to him, "the role of
the field is to provide a means for making observations of a
system of particles" and therefore "we cannot suppose the field
to be a dynamical system on the same footing as the particles and
thus be something to be observed in the same way as the
particles".

Based on such a philosophy, Dirac proposed
a new theory, the so-called many-time theory, which, besides
being a concrete example of his philosophy was of much more
satisfactory and beautiful form than other theories presented up
to then. In fact, from the relativistic point of view, these
other theories had a common defect which was inherent in their
Hamiltonian formalism. The Hamiltonian dynamics was developed on
the basis of non-relativistic concepts which make a sharp
distinction between time and space. It formulates a physical law
by describing how the state of a dynamical system changes with
time. Speaking quantum-mechanically, it is a formalism to
describe how the probability amplitude changes with time
t. Now, as an example, let us consider a system composed
of N particles, and let the coordinates of each particle
be r1, r2,...,
rN. Then the probability amplitude of the
system is a function of the N variables
r1, r2,...,
rN, and in addition, of the time t to
which the amplitude is referred. Thus this function contains only
one time variable in contrast to N space variables. In the
theory of relativity, however, time and space must be treated on
an entirely equal footing so that the above imbalance is not
satisfactory. On the other hand, in Dirac's theory which does not
use the Hamiltonian formalism, it becomes possible to consider
different time variables for each particle, so that the
probability amplitude can be expressed as a function of
r1, t1, r2t2,...,rNtN. Accordingly, the theory satisfies the
requirement of the principle of relativity that time and space be
treated with complete equality. The reason why the theory is
called the many-time theory is because N distinct time
variables are used in this way.

This paper of Dirac's attracted my interest
because of the novelty of its philosophy and the beauty of its
form. Nishina also showed a great interest in this paper and
suggested that I investigate the possibility of predicting some
new phenomena by this theory. Then I started computations to see
whether the Klein-Nishina formula could be derived from this
theory or whether any modification of the formula might result. I
found out immediately however, without performing the calculation
through to the end, that it would yield the same answer as the
previous theory. This new theory of Dirac's was in fact
mathematically equivalent to the older Heisenberg-Pauli theory,
and I realized during the calculation that one could pass from
one to the other by a unitary transformation. The equivalence of
these two theories was also discovered by Rosenfeld3 and by Dirac-Fock-Podolsky4 and was soon published in their
papers.

Though Dirac's many-time formalism turned
out to be equivalent to the Heisenberg-Pauli theory, it had the
advantage that it gave us the possibility of generalizing the
former interpretation of the probability amplitude. Namely, while
one could calculate the probability of finding particles at
points with coordinates r1,
r2,..., rN, all at the time
t according to the previous theory, one could now compute
more generally the probability that the first particle is at
r1 at time t1, the second at
r2 at time t2, … and
the N-th at rN at time tN. This was
first discussed by Bloch5 in
1934.

(2) In this many-time theory
developed by Dirac, electrons were treated according to the
particle picture. Alternatively, in quantum theory, any particle
should be able to be treated according to the wave picture. As a
matter of fact, electrons were also treated as waves in the
Heisenberg-Pauli theory, and it was well known that this wave
treatment was frequently more convenient than the particle
treatment. So the question arose as to whether one could
reformulate the Heisenberg-Pauli theory in a way which would be
more satisfactory relativistically, when electrons were treated
as waves as well as the electromagnetic field.

As Dirac already pointed out, the
Heisenberg-Pauli theory is built upon the Hamiltonian formalism
and therefore the probability amplitude contains only one time
variable. That is to say, the probability amplitude is given as a
function of the field strength at different space points and of
one common time variable. However, the concept of a common time
at different space points does not have a relativistically
covariant meaning.

Around 1942, Yukawa6 wrote a paper emphasizing this
unsatisfactory aspect of the quantum field theory. He thought it
necessary to use the idea of the g.t.f. (generalized
transformation function) proposed by Dirac7 to correct this defect of the theory.
Here I shall omit talking about the g.t.f., but, briefly,
Yukawa's idea was to introduce as the basis of a new theory a
concept which generalized the conventional conception of the
probability amplitude. However, as pointed out also by Yukawa, we
encounter the difficulty that, in doing this, cause and effect
can not be clearly separated from each other. According to
Yukawa, the inseparability of cause and effect would be an
essential feature of quantum field theory, and without abandoning
the causal way of thinking which strictly separates cause and
effect, it would not be possible to solve various difficulties
appearing in quantum field theory about which I will talk later.
I thought however, that it might be possible (without introducing
such a drastic change as Yukawa and Dirac tried to do) to remedy
the unsatisfactory, unpleasant aspect of the Heisenberg-Pauli
theory of having a common time at different space points. In
other words, it should be possible, I thought, to define a
relativistically meaningful probability amplitude which would be
manifestly covariant, without being forced to give up the causal
way of thinking. In having this expectation I was recalling
Dirac's many-time theory which had enchanted me 10 years
before.

When there are N particles in
Dirac's many-time theory, we assign a time t1
to the first particle, t2 to the second, and so
on, thus introducing N different times,
t1, t2,
...,tN, instead of the one common time
t. Similarly, I tried in quantum field theory to see
whether it was possible to assign different times, instead of one
common time, to each space point. And in fact I was able to show
that this was possible8.

As there are an infinite number of space
points in field theory in contrast to the finite number of
particles in particle theory, the number of time variables
appearing in the probability amplitude became infinite. But it
turned out that no essential difficulty appeared. An
interpretation quite analogous to the one discussed by Bloch in
connection with Dirac's many-time theory could be given to our
probability amplitude containing an infinite number of time
variables. Further, it was found that the theory thus formulated
was completely covariant and that this covariant formulation was
equivalent in its whole content to the Heisenberg-Pauli theory:
it was shown, just as in the case of the many-time theory, that
we could pass from one to the other by a unitary transformation.
I began this work about 1942, and completed it in 1946.

(3) As I mentioned a little while
ago, there are many difficulties in the quantum mechanics of
fields. In particular, infinite quantities always arise which are
associated with the presence of field reactions in various
processes. The first phenomenon which attracted our attention as
a manifestation of field reactions was the electromagnetic mass
of the electron. The electron, having a charge, produces an
electromagnetic field around itself. In turn, this field, the
so-called self-field of the electron, interacts with the
electron. We call this interaction the field reaction. Because of
the field reaction the apparent mass of the electron differs from
the original mass. The excess mass due to this field reaction is
called the electromagnetic mass of the electron and the
experimentally observed mass is the sum of the original mass and
this electromagnetic mass. The concept of the electromagnetic
mass had already appeared in the classical theory of the electron
by Lorentz, who computed the electromagnetic mass by applying the
classical theory and obtained the result that the mass becomes
infinite for the point (zero size) electron. On the other hand,
the electromagnetic mass was computed in quantum theory by
various people, and here I mention particularly the work of
Weisskopf9. According to him, the
quantum-mechanical electromagnetic mass turned out to be
infinite, and although the order of the divergence was much
weaker than in the case of the Lorentz theory, the observed mass,
which included this additional mass, would be infinite. This
would be, of course, contrary to experiment.

In order to overcome the difficulty of an
infinitely large electromagnetic mass, Lorentz considered the
electron not to be point-like but to have a finite size. It is
very difficult, however, to incorporate a finite sized electron
into the framework of relativistic quantum theory. Many people
tried various means to overcome this problem of infinite
quanties, but nobody succeeded.

In connection with field reactions, the
next problem which attracted the attention of physicists was
determining what kind of influence the field reaction exerts in
electron-scattering processes. Let us consider, as a concrete
example, a problem in which an electron is scattered by an
external field. In the ordinary treatment, we neglect the effect
of field reactions on the scattered electron, assuming that it is
negligibly small. Then the behavior of the scattering obtained by
calculation (e.g. the Rutherford formula) fits very well
with experiment. But what will happen if the influence of field
reaction is taken into account? This theoretical problem was
examined non-relativistically by Braunbeck-Weinmann10 and Pauli-Fierz11 and relativistically by
Dancoff12.

While Dancoff applied an approximation
method, the perturbation method, in his relativistic calculation,
Pauli and Fierz treated the problem in such a way that the most
important part of the field reaction was first separated out
exactly by employing a contact transformation method which was
similar to the one which Bloch-Nordsieck13 had published a year before. Since
Pauli and Fierz adopted a non-relativistic model, and further
simplified the problem by using the so-called dipole
approximation, their calculation was especially transparent. At
any rate, both non-relativistic and relativistic calculations
exhibited several infinities in the scattering processes*.

The conclusions of these people were fatal
to the theory. That is, the influence of the field reaction
becomes infinite in this problem. The effect of field reaction on
a quantity called the scattering cross section, which expresses
quantitatively the behavior of the scattering, rather than
becoming negligibly small, becomes infinitely large. This does
not, of course, agree with experiment.

This discouraging state of affairs
generated in many people a strong distrust of quantum field
theory. There were even those with the extreme view that the
concept of field reaction itself had nothing to do at all with
the true law of nature.

On the other hand, there was also the view
that the field reaction might not be altogether meaningless but
would play an essential role in the scattering processes, though
the appearance of divergences revealed a defect of the theory.
Heisenberg14, in his paper
published in 1949, emphasized that the field reaction would be
crucial in meson-nucleon scattering. Just at that time I was
studying at Leipzig, and I still remember vividly how Heisenberg
enthusiastically explained this idea to me and handed me galley
proofs of his forthcoming paper. Influenced by Heisenberg, I came
to believe that the problem of field reactions far from being
meaningless was one which required a frontal attack.

Thus, after coming back to Japan from
Leipzig, I began to examine the nature of the infinities
appearing in scattering processes at the same time that I was
engaged in the above-mentioned work of formulating a covariant
field theory. What I wanted to know was what kind of relationship
exists between the infinity associated with the scattering
process and that associated with the mass. If you read the
above-mentioned papers of Bloch-Nordsieck and Pauli-Fierz, you
will see that one of the terms containing infinite quantities is
first separated out by a contact transformation and this term
turns out to be just the term modifying the mass. Besides this
kind of infinity there appeared, according to Pauli-Fierz,
another kind of infinity characteristic of the scattering
process. I further investigated a couple of simple models which
were not realistic, but could be solved exactly. What was
understood from these models, was that the most strongly
divergent terms in the scattering process had the same form as
the expression giving the modification of the particle mass due
to field reactions, and therefore both should be manifestations
of the same effect. In other words, at least a portion of the
infinities appearing in the scattering process could be
amalgamated into the infinity associated with the particle mass,
leaving infinities proper to the scattering process alone. These
turned out to be more weakly divergent than the infinity
associated with the mass.

Since these conclusions were derived from
non-relativistic or unrealistic models, it was still doubtful
whether the same thing would occur in the case of relativistic
electrons interacting with the electromagnetic field. Dancoff
tried to answer this question. He calculated relativistically the
infinities appearing in the scattering process and determined
which of them could be amalgamated into the mass and which
remained as infinities proper to the scattering process alone. He
found that there remained, in the latter group of infinite terms,
one which was at least as divergent as the infinity of the mass,
a finding which differed from the conclusion based on fictitious
models.

Actually, there are two kinds of field
reactions in the case of the relativistic electron and
electromagnetic field. One of them ought to be called "of mass
type" and the other "of vacuum polarization type". The field
reaction of mass type changes the apparent electronic mass from
its original value by the amount of the electromagnetic mass as
was calculated by Weisskopf. On the other hand, the field
reaction of vacuum polarization type changes the apparent
electronic charge from its original value. As was discussed in
further papers by Weisskopf15 and others, infinite terms appear
in the apparent electronic charge if the effect of vacuum
polarization is taken into account. However, in this talk, for
simplicity, I will mention only briefly the divergence of the
vacuum polarization type.

(4) In the meantime, in 1946,
Sakata16 proposed a promising
method of eliminating the divergence of the electron mass by
introducing the idea of a field of cohesive force. It was the
idea that there exists unknown field, of the type of the meson
field which interacts with the electron in addition to the
electromagnetic field. Sakata named this field the cohesive force
field, because the apparent electronic mass due to the
interaction of this field and the electron, though infinite, is
negative and therefore the existence of this field could
stabilize the electron in some sense. Sakata pointed out the
possibility that the electromagnetic mass and the negative new
mass cancel each other and that the infinity could be eliminated
by suitably choosing the coupling constant between this field and
the electron. Thus the difficulty which had troubled people for a
long time seemed to disappear insofar as the mass was concerned.
(It was found later that Pais17
proposed the same idea in the U.S. independently of Sakata.) Then
what concerned me most was whether the infinities appearing in
the electron-scattering process could also be removed by the idea
of a plus-minus cancellation.

An example of a computation of how the
field reaction influences the scattering process was already
given by Dancoff. What we had to do was just to replace the
electromagnetic field by the cohesive force field in Dancoff's
calculation. I mobilized young people around me and we performed
the computation together18
Infinities with negative sign actually appeared in the scattering
cross-section as was expected. However, when we compared these
with the infinities with positive sign which Dancoff calculated
for the electromagnetic field, the two infinities did not cancel
each other completely. That is to say, according to our result,
the Sakata theory led to the cancellation of infinities for the
mass but not for the scattering process. It was also known that
the infinity of vacuum polarization type was not cancelled by the
introduction of the cohesive force field.

Unfortunately, Dancoff did not publish the
detailed calculations in his paper, and while we were engaged in
the above considerations, we felt it necessary to do Dancoff's
calculation over again for ourselves in parallel with the
computation of the influence of the cohesive force field. At the
same time I happened to discover a simpler method of
calculation.

This new method of calculation was to use
the technique of contact transformations based on the previously
mentioned formalism of the covariant field theory and was in a
sense a relativistic generalization of the Pauli-Fierz method.
This method had the advantage of separating the electromagnetic
mass from the beginning, just as was shown in their paper.

Our new method of calculation was not at
all different in its contents from Dancoff's perturbation method,
but had the advantage of making the calculation more clear. In
fact, what took a few months in the Dancoff type of calculation
could be done in a few weeks. And it was by this method that a
mistake was discovered in Dancoff's calculation ; we had also
made the same mistake in the beginning. Owing to this new, more
lucid method, we noticed that, among the various terms appearing
in both Dancoff's and our previous calculation, one term had been
overlooked. There was only one missing term, but it was crucial
to the final conclusion. Indeed, if we corrected this eror, the
infinities appearing in the scattering process of an electron due
to the electro-magnetic and cohesive force fields cancelled
completely, except for the divergence of vacuum polarization
type.

(5) When this unfortunate error of
Dancoff's was discovered, we had to reexamine his conclusions
concerning the relation between the divergence of the scattering
process and the divergence of the mass, in particular, the
conclusion that there remained a portion of the infinities of the
scattering process which could not be amalgamated into the
modification of the mass. In fact, it turned out that after
correcting the error, the infinity of mass type appearing in the
scattering process could be reduced completely to the
modification of the mass, and the remaining field reaction
belonging to the scattering proper was not divergent19. In other words, the highest
divergence part of the infinities appearing in the scattering
process, in the relativistic as well as in the non-relativistic
case, could be attributed to the infinity of mass. The reason why
the remaining part became finite in the relativistic case was due
to the fact that the order of the highest divergence was only log
co, and after amalgamating the divergence into the mass term, the
remainder was convergent. The great value of this method of
contact transformations was that once the infinity of the mass
was separated out, we obtained a divergence-free theoretical
framework.

In this way the nature of various
infinities became fairly clear. Though I did not describe here
the infinity of vacuum polarization type, this too appears in the
scattering process, as mentioned earlier. However, Dancoff had
already discovered that this infinity could be amalgamated into
an apparent change in the electronic charge. To state the
conclusion, therefore, all infinities appearing in the scattering
process can be attributed either to the infinity of the
electromagnetic mass or to the infinity appearing in the
electronic charge - there are no other divergences in the
theory.

It is a very pleasant thing that no
divergence is involved in the theory except for the two
infinities of the electronic mass and charge. We cannot say that
we have no divergences in the theory, since the mass and charge
are in fact infinite. It is to be noticed, however, that if we
reduce the infinities appearing in the scattering process to
modifications of mass and charge, the remaining terms all become
finite. Further, if we examine the structure of the theory, after
the infinities are amalgamated into the mass and charge terms, we
see that the only mass and charge appearing in the theory are the
values modified by field reactions - the original values and
excess ones due to field reactions never appear separately.

This situation gives rise to the following
possibility. The theory does not of course yield a resolution of
the infinities. That is, since those parts of the modified mass
and charge due to field reactions contain divergence, it is
impossible to calculate them by the theory. However, the mass and
charge observed in experiments are not the original mass and
charge but the mass and charge as modified by field reactions,
and they are finite. On the other hand, the mass and charge
appearing in the theory are, as I mentioned above, after all the
values modified by field reactions. Since this is so, and
particularly since the theory is unable to calculate the modified
mass and charge, we may adopt the procedure of substituting
experimental values for them phenomenologically. When a theory is
incompetent in part, it is a common procedure to rely on
experiment for that part. This procedure is called the
renormalization of mass and charge, and our method has brought
the possibility that the theory will lead to finite results by
the renormalization even if it contains defects.

The idea of renormalization is far from
new. Many people used explicitly or implicitly this idea, and we
find the word renormalization already in Dancoff's paper. In his
calculation it appeared, because of an error that there still
remained a divergence in the scattering even after the
renormalization of the electron mass. This error was very
unfortunate; if he had performed the calculation correctly, the
history of renormalization theory would have been completely
different.

(6) This period, around 1946-1948,
was soon after the second world war, and it was quite difficult
in Japan to obtain information from abroad. But soon we got the
news that in the U.S., Lewis and Epstein20 found Dancoff's mistake and gave the
same conclusions as ours, Schwinger21 constructed a covariant field theory
similar to ours, and he was probably performing various
calculations making use of it. In particular, little by little
news arrived that the so-called Lamb-shift was discovered22 as a manifestation of the
electromagnetic field reaction and that Bethe23 was calculating it theoretically. The
first information concerning the Lamb-shift was obtained not
through the Physical Review, but through the popular
science column of a weekly U.S. magazine. This information about
the Lamb-shift prompted us to begin a calculation more exactly
than Bethe's tentative one.

The Lamb-shift is a phenomenon in which the
energy levels of a hydrogen atom show some shifts from the levels
given by the Dirac theory. Bethe thought that the field reactions
were primarily responsible for this shift. According to his
calculation, field reactions give rise to an infinite level
shift, but he thought that it should be possible to make it
finite by a mass renormalization and a tentative calculation
yielded a value almost in agreement with experiments.

This problem of the level shift is
different from the scattering process, but it was conceivable
that the renormalization which was effective in avoiding
infinities in the scattering process would be workable in this
case as well. In fact, the contact transformation method of Pauli
and Fierz devised to solve the scattering problem could be
applied to this case, clarifying Bethe's calculation and
justifying his idea. Therefore the method of covariant contact
transformations, by which we did Dancoff's calculation over again
would also be useful for the problem of performing the
relativistic calculation for the Lamb-shift. This was our
prediction.

The calculation of the Lamb-shift was done
by many people in the U.S.24.
Among others, Schwinger, commanding powerful mathematical
techniques, and by making thorough use of the method of covariant
contact transformations, very skilfully calculated not only the
Lamb-shift but other quantities such as the anomalous magnetic
moment of the electron. After long, laborious calculations, less
skilful than Schwinger's, we25
obtained a result for the Lambshift which was in agreement with
Americans'. Furthermore, Feynman26 devised a convenient method based on
an ingenious idea which could be used to extend the approximation
of Schwinger and ours to higher orders, and Dyson27 showed that all infinities appearing
in quantum electrodynamics could be treated by the
renormalization procedure to an arbitrarily high order of
approximation. Furthermore, this method devised by Feynman and
developed by Dyson was shown by many people to be applicable not
only to quantum electrodynamics, but to statistical mechanics and
solid-state physics as well, and provided a new, powerful method
in these fields. However, these matters will probably be
discussed by Schwinger and Feynman themselves and need not be
explained by me. So far I have told you the story of how I played
a tiny, partial role in the recent development of quantum
electrodynamics, and here I would like to end my talk.

* The main purpose of the
work of Bloch-Nordsieck and Pauli-Fierz was to solve the
so-called infrared catastrophe which was one of a number of
divergences. Since this difficulty was resolved in their papers
we confine ourselves here to a discussion of the other
divergences which are of the so-called ultraviolet type.